1. Field of the Invention
The present invention relates to a communication apparatus, a network system, a communication method applied to a network system and a communication apparatus, and a program for causing a computer to implement the communication method.
2. Description of the Related Art
Conventionally, as a system for simultaneously distributing data to a plurality of users, the server-client type system has been the mainstream in which a server for performing centralized management of information resources such as a database is arranged and the server distributes the information resources it manages to users, that is, the clients of the server. However, this method has some problems such as the problem that the load is concentrated on the server and the problem that installation of the server requires a cost.
It is a peer-to-peer network that was devised in order to solve such problems. The peer-to-peer network is a virtual network constructed by connecting node apparatuses with each other not via a server. In this peer-to-peer network, it is possible to exchange information with an unspecified large number of partners (node apparatuses) (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2006-221423).
However, in a peer-to-peer network (virtual network), a node apparatus not participating in the peer-to-peer network cannot acquire node information about the node apparatuses on the peer-to-peer network at all. Therefore, it has been a problem of the peer-to-peer network how a node apparatus not participating in the peer-to-peer network can find a node apparatus participating in the peer-to-peer network.
There is a method in which, in order to find a node apparatus participating in a peer-to-peer network, a particular server is used as an access point only at the time of participating into the peer-to-peer network. However, this method requires that the particular server should be provided, and the cost therefor is required.
There is also a method in which connection to the node apparatus connected at the last participation into a peer-to-peer network is attempted again. However, in this method, it is not possible to participate in the peer-to-peer network if the node apparatus has already separated from the peer-to-peer network.